Mullite-bonded zircon pebbles



Patented Apr. 7, 1953 UNITED STATES PATENT OEFF'ICE "masses "MULLITE-BON-DED ZIRCON 'PEBBLES Sam P. Robinson, Bartlesville,"Okla assignor to Phillips Petroleum Company, a corporation of Delaware 'Nonrawin' Application .Tiilyli, 1 949,

' 'Siiil'NO. 1041157 .Pebble heater techniques being developed and i appliedto various gas heating, treating, and reaction processes'at the present tirne make use 'of""a-'-c'ompact stream or small refractory pebbles as'amov'ingheatexcharigemedium. These pebbles which "are usually ceramic materials, --a=l-- though the'ymay be -meta1lic -for* some applications, arespheres ranging in size from about %"to 1 in diameter. 'They maybe either ''atalytic or non-catalyticin' -a given application. In typical pebble heater operation, "a continuous compact mass of pebbles descendsby gravity thr-u a'series of treating zones-and upon emerging from the-lowermost zone, they are-elevated by asuitable elevator, :usually of the bucket type, to a point-above the uppermost zone for againgravitating thru the system. The uppermost zone is usually-a pebble "heatingaonewhere the'pebbles "are contacted in "countercurr'ent fiow "with a stream of hot combustion gasso as to raise ;their "temperature to a desireddegree as they descend thru the heating zone. The heated pebbles then *pass into a reaction or'g a' s heating zone where th'ey impart heat to --the gas being I treated and inturn are cooled and require reheating. In some installations, a feed gas prehea-ting "zone is positioned just-below the reaction r*-gas-. treating z'one so as to further coollt-he l pebblesfbefore elevation and to preheatthe feed -gas to the-reaction 'zon'e. ""Othe'r installations 'utilize'a pebble preheating zone positionedldirectly above the pebble heating zone proper where the-pebblesarecon- 'tacted'vvith the effluent from the reaction zone so as to recover a substantial portion of the-sen- "s'ible heat thereof and simultaneously quench the reaction product.

'Imanothertype of pebble heat-exchange process, agravitating mass of pebbles is utilized to maintain -a cold zone or to cool a gas. The peb- -bles-' are cooled by contact with a cold-gas inone 2 relating to the-cracking of hydrocarbons to hydrogen and coke, as well-as theprocessof thecopending-application of M. O. Kilpat-rick, i Serial No. 76'1',696,-fi1ed July '17, 1-947, relating to the chamber and the cold pebbles are then gravitated .45

thermal conversion of hydrocarbons vto more edezsirable hydrocarbons. These processes involve temperature changes of the :order of 1000" Lto -2000 I. per minute, with ,severe mechanical shock and abrasive forces present.

The pebble. heater finds its greatest utility operations which require extremely .ifas't'. heating rates and therefore extremely fast pebble coming --rateswith concomitant thermal. shoeklto'thepebrbles. In l-pebble. heater processes ,invblving more severe-heating and-cooling requirements, the pebbles are subjected to .heating rates grzitfj'tha'n --1000 per minute .and cooling rates jof more than 20Q0.F..; per minute at maximmntempera- 'pr oper 'size have been i formed, :-:are found pto sexhibit laminar structure and suffer breakage: under the strain'of pebble heater operatin'gzconditions. PebbIes which are made by slugging :and oompacting the slugs into spheres do not exhibihthis laminar structure andiare much morerresistanta-to breakage under pebble heater operation :.conditions. However, it has *been found "that even when pebbles have been made by slug'ging rand *compacting the slugs into balls, they :mus't E be fired at a temperature within a criticalran'gei-in order to pr'operly'bond thepebble*crystals and "produce a pebble which is ru'gg'ed under s'evere conditions of service. The critical temperature range .for ifiring v aaspecific pebble depends upon the ingredients oftheipebble, .-e.jg., "the critical range for 'firin'g high purity "alumina pebbles is -3000 to 3150" 'F.-as 'disclosed-in' my' copending -application, Serial No. 23,245, filed April 26,1948, eand 'a zirconia-stabilized. alumina-pebble containing from 0.5 to @lO'Weight percent-zirconium .oxide requires :a firing temperature imtheran'ge 3 of 2950 to 3250 F. as disclosed in my copending application, Serial No. 53,590, filed October 8, 1948.

The use of zircon (ZrOz-SiOz or ZrSiOr) as a pebble material has been considered to afiord exceptional possibilities in the pebble heater field but up to the present time no method of forming a strong bond in a high zircon content pebble has been found. Zircon has a melting point in the neighborhood of 4700 F. and when bonded properly so as to resist breakage and abrasion in pebble heater use, pebbles of this material have exceptional utility in high temperature pebble heater processes. The chief difficulty in sintering zircon to a strong bond lies in the fact that this mineral does not sinter at temperatures below about 3500 F. and at temperatures around 3200 F. it begins to decompose into ZrOz and SiOz. This silica thrown down at this temperature is very deleterious in a pebble if uncombined with other minerals as a crystal or if fiuxed with impurities to form a glass.

The principal object of the present invention is to provide a strong, rugged, zircon pebble having high resistance to breakage and attrition under severe cyclic thermal and mechanical shock conditions. Another object of the invention is to provide a method of manufacturing a strongly bonded high zircon content pebble. It is also an object of the invention to provide a method of' forming a strong bond in a zircon pebble. A further object of the invention is to provide a method of heat treating a zircon-alumina-aluminum silicate raw pebble so as to produce a mullitebonded zircon pebble. A still further object of the invention is to provide improved heat-exchange processes effected in the presence of gravitating mullite-bonded zircon pebbles. Other objects of the invention will become apparent from a consideration of the accompanying disclosure.

The invention is concerned principally with the bonding of zircon into a strong, rugged pebble having high resistance to breakage, attrition, and fluxing. It is found that by incorporating in a high purity zircon an aluminum silicate convertible to mullite at high temperatures and, in

addition, enough free alumina. to react with the free silica thrown down when the aluminum silicate is converted to mullite according to the equations and forming it into pebbles which are then heat treated at a temperature in the range of 2600 to 3100 F. so as to convert the aluminum silicate to mullite and react the free silica thrown down with the free alumina of the pebble, a strongly bonded zircon pebble is formed which has excellent breakage, attrition, and fiux resistant properties at high temperatures in pebble heatexchange type apparatus. From a consideration of the foregoing equations it can be readily seen that these aluminum silicates in which the ratio of silica to alumina is 1:1 or 2:1 convert to mullite in the presence of sufficient free alumina according to the following equations:

(3) 3A12O3+6 (A12O3.Si02) =3 (3A12Oa2SiO2) (4) 6A1203+3 (A12O3.2SiO2) =3 (3A12O3.2Si02) By incorporating sufiicient uncombined alumina in the raw pebble to combine with the silica freed by the conversion of the aluminum silicate to mullite when the pebble is fired to bond the zircon with mullite, the formation of silica and/or glass in the finished pebble is avoided and a strong rugged, breakage resistant pebble is formed.

The process of the invention in its broadest form entails forming a homogeneous mixture of high purity zircon and aluminum silicate together with at least sufilcient uncombined alumina to react with all of the free silica formed upon conversion of the aluminum silicate to mullite and compacting the mixture into balls with the aid of a binder, such as water and/or a volatilizable organic binder and lubricant, such as Sterotex (hydrogenated corn oil), any of the synthetic or natural resins (including wood rosin), petroleum pitch, stearic acid, aluminum stearate, carboxymethyl cellulose, starch, fiour, molasses, sugar, dextrin, shellac, glue, etc. The zircon may be anyhigh purity mineral of this composition, such as commercial zircon sand of high grade and should be comminuted to pass a lilo-mesh screen and preferably a -325-mesh screen. The aluminum silicate used is preferably a plasticizing ball clay but may be any of the aluminum silicates, or mixtures thereof, convertible to mullite at high temperatures and these include kaolin, andalusite, kyanite, sillimanite, as well as ball clay. Andalusite, kyanite, and sillimanite all have the composition Al2O3.SiO2, and convert into mullite at temperatures of approximately 2460' F., 2415 F., and 2786" R, respectively. Kaolin and ball clays have the composition A12O3.2S1O2 and are converted to mullite at slightly lower temperatures. Mullite itself decomposes at approximately 3325 F. to alpha corundum and silica, and therefore under no circumstances should the pebble be heat treated at this temperature or higher because of the danger of converting the mullite binder to alumina and silica. The use of plastic ball clay is preferred because this type of clay imparts sufficient plastic properties to the mixture of zircon, clay, and alumina when mixed with from 10 to 20 per cent water to form a readily extrudable or compactable mix for the pebble forming operation. When using the less plastic silicates it is sometimes necessary to include in the pebble mix from 2 to 10 weight per cent of one or more of the volatilizable organic binders hereinbefore specified, or sufi'icient ball clay to impart plastic properties to the mix.

The pebbles or balls may also be compacted from a homogeneous plastic mix of the constitdents and organic binder Without the use of added Water in which case from 3 to 15 per cent, preferably from 5 to 10 per cent, of the binder is added. In this method of manufacture heat may be applied to the mix in order to facilitate the pebble forming step.

Another pebble forming method involves starting with a small nucleus A, or smaller in diameter consisting of zircon and mullite prepared by firing a mixture of zircon and aluminum silicate convertible to mullite and alumina, or of these constituents themselves prior to firing, and applying successive layers of a powdered mixture of the pebble raw materials using a tacky binder of the class described, particularly dextrin, molasses, sugar, glue, shellac, etc. The core can readily be made by nodulizing or by the cottage cheese method of forming small balls.

The invention in its broadest sense does not require any specific pebble forming step although it is preferred to form the pebbles from an extrudable plastic mix of the pebble constituents by extruding and cutting the extrudate into short slugs and thereafter compacting the sIiigs *rnto spheres by "tumbling in a "three dimensional type cylindrical tumbler, because this m'thod pro- 'duces superior pebbles in'attritionand fracture resistance in cyclic pebble heat-transfer -equipm'ent. Any-method of compacting thepebble "constituents into a homogeneous, dense ball or sphere is suitable and within the -s'cop'e of invention.

The alu'mi'n'a f or the pebbles of the invention may 1 be incorporatedin the the torm of "alpha corundum, g-amm'a alumina, an bf the hydrated aluminas which are converted to al-pha corundum by heating to elevated temperatures, qrammimim hydroxide. It is desirable thatithe alumina 'be ef high purity, such asee per cent and r'ifeferably 99i5 -per cen't. Purified bauxite, precipitated 1 aluminum hydroxide, and the amm-mafmanutacturedin the Bayeripro'ces's are ex- :amples otsuitablerawmaterials for: theialumina. A preferred alumina is the finely divided "precalcine'd 'crypto crystalline alpha 'corundum 'for med by calcining -Bayer iprocess alumina at temperaturesaround -21 00 The alumina raw material must be :finely oomm inuted-such-as to pass a lOO-mesh screen, and 'preferably in the rangeof 1:50 to 625 meslr so as to'form coherent balls in which the alum'ina and aluminum 'sil-ica't'e are intimately associated and which are of exoeedingly-finegrain.

The .pebble mix "should comprise l'a'lmajor proportion of zircon and minoriprop'ortions of one or more aluminum :silicatesl and alumina; the last being 'pre'sentepreferably:inflan'samount iof from 1 to 25 mol per cent in excess of that required to combine with all of the free silica thrown down 'when'the aIuminum silicate is converted to mullite. Itis preferred taincorporatefrom '75 to 98 weight per cent of I zircon in: the pebble mix, the balance consisting of aluminum :silicat'e convertible to mullite, and sufiicient ifree uncombined alumina toreact with all 'of the silica freed by conversion to mullite (weights based on' solid constituents in the :mix exclusive of binder and/or lubricant). The incorporation of alu- 'mma: in -th'e r'a-w p'e'bble in a maximum-"amount "of 25 mol per cent ex'cess gives a final pebble-of about 3w'eig ht per cent alumina when-the pebble contains 75 weight-per ce'nt zircon. More alumin'athan this decreases the breakageresistance :of thefp'ebble and is to he avoided. The water end/or other binder in the mix-impart coherence t'o'-'the' pebbles or balls prior to the or calcinin'g 'stepat which time they a're removed from the' pebbleby' volatilization. 'While it is preferred to incorporate alumina in "excess of that required toreact with all of the silica therein-upon conversion of aluminum silicate to mullite as a precaution against an insufficiency of alumina with resulting free silicain the final-pebbla the .Ipebble of the invention may consist entirely of zirconandmullite without either free silica or free alumina being. present.

In compacting pebbles according tea-preferred modification of the I invention, a homogeneous, "aqueous, plastic mix is formed by "mixing the finely comminuted raw materials with a .suffi- -cientamount of water to form an "extrudable "mixture. The mixture is thoroughly'plasticized in :a mulling pan mixer with sufficient water to produce a mix containing between .10 'and 20 weight per cent water which has thel propercon- --sistencyfor'extru'sion. The water content-may be: adjusted either :up-lor down at any tim'e nur- "in several'ways. machine utilizin three-dimensional rotation in acetate within'the rangetest-recited so as' tofobtain the proper consistency for extrusion. The homose- "theextruded rodsare not completeiyhomogeneoi'i's str-ucture andwill i esult in -the formation of an inferior pebble. I If i the moisture content "exceeds 20 per cent, -the "extruded rods "are too 'sticky and' -thefslugs cannot be properly handled in the subsequent ballin step. For 'best per- "formanceduring-extrusion, a moisture content between about '16 Band 18 percent by weight is desirable. =When making ""pebbles,-i extrusion of the plastic 'mix into "rods and cutting them into lengths permits the compacting of -'slu'gs"whi'ch will be of the 'desiredcsize 7 after fir-- ing. High-pressure extrusion or this type, preferably-in a piston typepressiwith'or without deairing of the feed, is preferred to other methods of preparing -the slugs"'for the balling operation,

inasmuch asa homogeneous bod-y results with minimum variations inistructure a'fter 'firing, to-

gether with avoidance of laminar str'uoture.

"However, other methods of *preparing the --'sl'ugs are Within the scope of theinv'ention.

Following the-cutting of the extruded mix into "slugs, the slugs are prefer-ably driedto a moisture content between about 1'0 'and '14 *per cent by weight before compacting or rolling into balls, the next step of the operation. Wetter slugs'tend to ball up and stick together, while'dry'slugs'roll upinto'ballsfwhi'ch develop internal cracks upon firing. A preferred moisture content for this step "lies"'between 11 and 12;5 per cent. Compacting of the'slug's into ballsor pebbles can be performed Rollingof theslugs in a balling a cylindrical "drum placed at "angles to all three axes of conventional rotary equipment is found "to'make'the 'most suitable pebbles upon firing. The balls are morefirmlycompacted and more nearly spherical in shape'th'an when made by any other known method. This is probably due :to

the'fact thatthe slugsare rolld in all directions 'duringthe rolling .orcompacting step. The resulting spherical pebbles containing the proper =swept'to eliminate the predrying stop.

After the balling istepl it: is necessary to thoroughly but-slowly dry' the balls to less than 'one weight per cent 'm'oistu're before firingto high temperatures. This can be accomplished in "a variety of commercial dryers. slowness indry- 'ing is required to insure removal of moisture from the center core of the pebbleand'toprevent cracked internal structure'from too fast heating and drying in the subsequent calcining step. Thetemperature in1the dryer -shou1d not exceed sing -the; plasticizing :and I homogenizing ss't'ep {=7 :aboutAOU" AF. iihtil the nieisture content of 'tl'ie and unavailable porosity).

balls is reduced to approximately 1 or 2 weight per cent.

The critical firing temperature of the compacted balls of zircon, aluminum silicate, and alumina lies in the range of 2500 to 3100 F. with an optimum range of 2800 to 3000 F. When the pebbles are fired at lower temperatures, the bond is apparently not sufficiently developed and when firing above this range, the pebbles are apparently too hard and rigid under conditions of cyclic thermal and mechanical shock and have poorer attrition resistance in pebble heater operation. Firing in the above range must be continued for at least 3 hours and until the porosity of the pebbles lies in the range of 8 to 15 per cent, and preferably 9 to 12 per cent (both available The proper time for firing the pebbles depends upon the temperature, shorter times being required at the higher temperatures within the firing range. At a temperature of 3000 F. at least 3 hours is required to effect the conversion of aluminum silicate to mullite and react the free silica formed with alumina to produce a composition of zircon, mullite, and alumina, while at a temperature of 2500 F. a time of 60 hours is required to bring about the required conversion and to produce a pebble having a porosity in the range specified. At a temperature of 2800 F. a firing period of 36 hours is required, and at 2900 F. a firing period of at least 12 hours is required. Pebbles fired at a temperature in the range of 2800" to 8000 F. are better pebbles from the standpoint of attrition and breakage resistance in pebble heater service.

When hydrated alumina, gamma alumina, or aluminum hydroxide, or mixtures thereof, are included in the raw pebble composition they are gradually converted to alpha alumina (corundum) as the treatment of the pebble progresses at elevated temperatures upwards of 1800 F.

The finished pebble of the invention is an intimate homogeneous mixture of zircon, mullite, and alumina (when sufficient alumina is incorporated in the raw pebble to react with all of the silica thrown down and provide an excess of alumina). The crystals of zircon, mullite, and alumina are present in an interlocking uniform dispersion or arrangement therein which results in a rugged and dense composition with an exceedingly fine and smooth texture, which characteristics are believed to provide the pebbles with desirable attrition and breakage resistance in cyclic pebble heater processes.

Firing or calcination of the pebbles can be suitably effected in any conventional equipment which results in maintaining the entire mass of pebbles at a relatively even temperature within the specified range. Firing in continuous shaft kilns produces pebbles which are inferior in service in pebble heater type apparatus because they are not uniformly heated in all parts of the bed, a large proportion being either underfired or overfired. The former are not strong and stand up poorly to heat and mechanical shock and attrition, while the latter are too rigid and soon develop cracks along large crystal faces, resulting in early breakage in service as well as low resistance to attrition.

The following example offers a specific illustration of one modification of the invention:

Example An intimate homogeneous mix is formed consisting of 1780 pounds of 325 mesh commercial zircon sand of high purity, 67 pounds of-325 mesh ball clay substantially free from iron compounds and other deleterious materials, 6'7 pounds of 325 mesh Bayer process alumina of 99.5 per cent purity, and 375 pounds of water (approximately 16 weight per cent of the mix). The mix is extruded into rods in a piston type extrusion press and the rods are automatically cut into slugs. The slugs are simultaneously dried to a moisture content of 11.5 weight per cent and compacted into balls by tumbling in a threedimensional tumbling drum thru which a low temperature stream of fiue gas is passed during the tumbling. The halls are then dried to a moisture content below 0.5 per cent in conventional fiue gas driers at a gas temperature of 300 F. after which they are brought to a temperature of 2900 F. over a 16 hour period in a periodic kiln, fired at this temperature for 12 hours, and gradually cooled to atmospheric temperature over a 24 hour period.

The resulting pebbles are approximately 1 5'" in diameter and have a hard, dense, apparently glazed surface. Their porosityis approximately 11 per cent. These pebbles have a crushing strength averaging well over 1200 pounds and stand up exceptionally well in heat-shock and attrition tests compared to the best commercially available pebbles, being more than 3 times more resistant to breakage and 2.5 times more attrition resistant.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

I claim:

1. A process for manufacturing pebbles for moving-bed heat-transfer purposes which comprises compacting into small dense balls a homogeneous finely comminuted plastic mixture consisting essentially of a major proportion of zircon, and the balance consisting essentially of a plastic aluminum silicate in which the mol ratio of silica to alumina is in the range of 1 :1 to 2:1 and alumina in an amount in the range of 1 to 25 weight per cent in excess of that required to react with the silica freed when said aluminum silicate is converted to mullite by calcination; and calcining the balls at a temperature in the range of 2500 to 3100 F. for at least 3 hours and until their porosity lies in the range of 8 to 15 per cent so as to convert aluminum silicate to mullite and react freed silica with alumina to form additional mullite in the balls as bonding agent therefor.

2. The process of claim 1 in which the zircon in the mixture is in the range of 75 to 98 weigh per cent of the solid constituents thereof.

3. A process for manufacturing pebbles for moving-bed heat-transfer purposes which comprises forming a homogeneous finely comminuted plastic aqueous mix consisting essentially of 75 to 98 weight per cent zircon (based on the Weight of solid), and the balance consisting essentially of a plastic aluminum silicate in which the mol ratio of silica to alumina is in the range of 1:1 to 2:1 and 1 to 25 weight per cent alumina in excess of the amount required to react with the silica freed when said aluminum silicate is converted to mullite in the hereinafter specified calcination step; adjusting the water content of said mix to the range of 10 to 20 Weight per cent; compacting said mix into dense 3 to 1" balls; slowly drying said balls so as to reduce the moisture content below 1 weight per cent; and calcining the dried balls at a temperature in the range 9 of 2500 to 3100 F. for at least 3 hours and until their porosity lies in the range of 8 to 15 per cent so as to convert aluminum silicate to mullite and react freed silica with alumina to form additional mullite in the balls as bonding agent therefor.

4. The process of claim 3 in which the compacting step comprises extruding said mix into rods to I" in diameter, cutting the rods into slugs of a length approximating their diameter, and compacting the slugs into balls by tumbling.

5. A process for manufacturing pebbles for moving-bed heat-transfer purposes which comprises forming a homogeneous finely comminuted aqueous mix consisting essentially of '75 to 98 per cent zircon (based on the weight of solid), and the balance consisting essentially of a plastic aluminum silicate in which the mol ratio of silica to alumina is in the range of 1:1 to 2:1 and 1 to 25 weight per cent alumina in excess of the amount required to react with the silica freed when said aluminum silicate is converted to mullite in the hereinafter specified calcination step, said ingredients being of at least 100 mesh fineness; adjusting the water content of said mix to the range of 15 to 20 weight per cent; extruding said mix into rods /8" to 1" in diameter, cutting the rods into slugs of a length approximating their diameter; drying the slugs to a moisture content in the range of 10 to 14 weight per cent; compacting the partially dried slugs into dense to 1" balls; slowly drying said balls at a temperature below 400 F. so as to reduce their moisture content to less than 1 weight per cent; and calcining the dried balls at a temperature in the range of 2500 to 3100 F. for a, time in the range of '3 to 60 hours and until their porosity lies in the range of 8 to 15 per cent so as to convert aluminum silicate to mullite and react freed silica with alumina to form additional mullite in the balls as bonding agent therefor.

6. A method of heat treating raw pebbles consisting essentially of 75 to 98 weight per cent zircon, and the balance consisting essentially of aluminum silicate convertible to mullite upon calcination in the hereinafter specified temperature range, and 1 to 25 weight per cent alumina in excess of the amount required to react with the silica freed when said aluminum silicate is converted to mullite in the hereinafter specified calcination step, which comprises calcining said pebble at a temperature in the range of 2500 to 3100 F. for a time in the range of 3 to hours and until the porosity of thepebble lies in the range of 8 to 15 per cent so as to convert aluminum silicate to mullite and react freed silica with alumina to form additional'mullite in the balls as bonding agent therefor.

7. A mullite-bonded zircon pebble consisting of to 98 weight per cent zircon, at least 2 weight per cent mullite, and not more than 3 weight per cent of alumina, said pebble having been made by the process of claim 1.

8. A mullite-bonded zircon pebble consisting of '75 to 98 weight per cent zircon, at least 2 weight per cent mullite, and not more than 3 weight per cent alumina.

9. A mullite-bonded, dense, smooth zircon pebble consisting of from 75 to 98 weight per cent zircon, at least 2 weight per cent mullite, and not more than 3 weight per cent alumina, the mullite crystals thereof being in intimate interlocking arrangement with the zircon crystals.

SAM P. ROBINSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,303,304 Schleicher Nov. 24, 1942 2,399,225 Heany Apr. 30, 1946 2,429,545 Bergstrom Oct. 21, 1947 2,463,979 Langrod Mar. 8, 1949 2,486,627 Arnold Nov. 1, 1949 

1. A PROCESS FOR MANUFACTURING PEBBLES FOR MOVING-BED HEAT-TRANSFER PURPOSES WHICH COMPRISES COMPACTING INTO SMALL DENSE BALLS A HOMOGENEOUS FINELY COMMINUTED PLASTIC MIXTURE CONSISTING ESSENTIALLY OF A MAJOR PROPORTION OF ZIRCON, AND THE BALANCE CONSISTING ESSENTIALLY OF A PLASTIC ALUMINUM SILICATE IN WHICH THE MOL RATIO OF SILICA TO ALUMINA IS IN THE RANGE OF 1:1 TO 2:1 AND ALUMINA IN AN AMOUNT IN THE RANGE OF 1 TO 25 WEIGHT PER CENT IN EXCESS OF THAT REQUIRED TO REACT WITH THE SILICA FREED WHEN SAID ALUMINUM SILICATE IS CONVERTED TO MULLITE BY CALCINATION; AND CALCINING THE BALLS AT A TEMPERATURE IN THE RANGE OF 2500* TO 3100* F. FOR AT LEAST 3 HOURS AND UNTIL THEIR POROSITY LIES IN THE RANGE OF 8 TO 15 PER CENT SO AS TO CONVERT ALUMINUM SILICATE TO MULLITE AND REACT FREED SILICA WITH ALUMINA TO FORM ADDITIONAL MULLITE IN THE BALLS AS BONDING AGENT THEREFOR. 